Bugfix: Frequencies always scaled by 1e9

Fixes a frequency scaling bug when a network is created using property setters. Network(f=[1,2,3],s=[1,2,3],z0=50,f_unit='hz') results in a Network object with a Frequency property spanning 1..3 GHz because the unit was not read in during construction until after the frequency array was already scaled by the default 'ghz' unit scaling.
Merge pull request #42 from Keysight-NBrennan/patch-1

Fix frequencies always scaled by 1e9